Particle emission standards of the EU 4 exhaust gas standard (0.05 g/km) can be met by heavy vehicles only with diesel particle filters (DPFs). DPF systems typically cut the emitted particles by 90–95%. The particles that become deposited in the filter as a result increase the exhaust gas back pressure, so that the diesel particle filter has to be regenerated at intervals between 200 and 500 km. The regeneration is accomplished by burning off (oxidizing) the deposited particles. This typically requires the particles to be heated to around 600° C. It is practical for the heating of the particles to be accomplished by convective input of heat through the exhaust gas stream. However, the temperature of the exhaust gas stream of diesel engines optimized for fuel consumption (TDI, CDI) only exceeds 300° C. at a few operating points. The exhaust gas therefore has to be heated additionally during the regeneration. This can be done electrically or by using a burner. Since the residual oxygen content of the exhaust gas fluctuates between 3% and 18%, using a diesel burner in the direct exhaust gas stream without an additional fresh air blower is problematic, since there is not sufficient oxygen available at all times to burn the fuel.
Conventionally, the ignition temperature of the particles is lowered to around 350° C. by adding organometallic iron or cerium compounds to the diesel fuel as additives. However it must be remembered in that case that such additives leave inorganic ash in the particle filter, resulting in a continuous rise in the back pressure produced by the diesel particle filter, which may make early replacement of the filter necessary.
Conventionally, electrically heatable diesel particle filters in partial stream or full stream solutions. In full stream systems, during the regeneration the entire exhaust gas stream is passed through the diesel particle filter and electrically heated. Such full stream systems do without switchable flaps, and can be manufactured relatively inexpensively and compactly. A disadvantage of such solutions, however, is that the entire mass flow of exhaust gas has to be heated above the ignition temperature of the diesel particulate. As an example, let us assume a piston displacement of 2.5 liters, an engine speed of 2000 rpm, and a boost pressure of 1.4 bar. This produces an exhaust gas flow of 250 kg/h. To heat this typically obtained mass flow by 400 K, the minimum heating power, ignoring losses, is 33 kW. Since a maximum of 2–2.5 kW of electrical heating power is implementable with a 12-volt on-board electrical system, partial stream solutions are generally preferred. A conventional partial stream system is shown in FIG. 1. Two diesel particle filters 1, 2, connected in parallel with each other, are recognizable. A flap 4 is inserted into the exhaust gas supply line 3 of these diesel particle filters, by which the exhaust gas in supply line 3 can be introduced optionally through a supply line 3a into diesel particle filter 1 or through a supply line 3b into diesel particle filter 2. Diesel particle filters 1, 2 each have electric heaters 1a, 2a. Fresh air may be introduced into supply lines 3a, 3b by a blower 5. Exhaust gas emerging from diesel particle filters 1, 2 is carried away through discharge lines 6a and 6b, respectively, which lead into a line 6.
With a system of this sort, it is practical for the diesel particle filters to be subjected to regeneration individually.
For example, during regeneration of diesel particle filter 1, the bulk of the exhaust gas stream (for example 90%) is routed by flap mechanism 4 through diesel particle filter 2. The remainder of the stream is heated electrically, or also by fossil fuel, and heats diesel particle filter 1 and the diesel particulate which is deposited there. If the residual oxygen content of the exhaust gas stream is too low, fresh air can be fed in by blower 5. However, the maximum pressure buildup of the blower, typically up to 150 hPa, limits its use to relatively small overpressures in the exhaust gas tract. The magnitude of the partial stream can be adjusted or dimensioned so that diesel particle filter 1 is heated above the ignition temperature of the diesel particulate in a short time, using the maximum implementable electrical heating power.
After the regeneration of diesel particle filter 1 has ended, diesel particle filter 2 can be regenerated. It is also possible to provide phases between the regeneration of the individual diesel particle filters in which exhaust gas is sent to both diesel particle filters equally, corresponding to normal operation.